1. Field of Invention
The present invention relates generally to optical imaging systems that monitor oxygen levels in tissue. More specifically, the present invention relates to optical probes that include sources and detectors that are not symmetrically arranged on sensor heads of the optical probes.
2. Description of the Related Art
Near-infrared spectroscopy has been used for non-invasive measurement of various physiological properties in animal and human subjects. The basic principle underlying the near-infrared spectroscopy is that physiological tissues include various highly-scattering chromophores to the near-infrared waves with relatively low absorption. Many substances in a medium may interact or interfere with the near-infrared light waves propagating therethrough. Human tissues, for example, include numerous chromophores such as oxygenated hemoglobin, deoxygenated hemoglobin, water, lipid, and cytochrome, where the hemoglobins are the dominant chromophores in the spectrum range of approximately 700 nm to approximately 900 nm. Accordingly, the near-infrared spectroscope has been applied to measure oxygen levels in the physiological medium such as tissue hemoglobin oxygen saturation and total hemoglobin concentrations.
Various techniques have been developed for the near-infrared spectroscopy, e.g., time-resolved spectroscopy (TRS), phase modulation spectroscopy (PMS), and continuous wave spectroscopy (CWS). In a homogeneous and semi-infinite model, both TRS and PMS have been used to obtain spectra of an absorption coefficient and reduced scattering coefficient of the physiological medium by solving a photon diffusion equation, and to calculate concentrations of oxygenated and deoxygenated hemoglobins as well as tissue oxygen saturation. CWS has generally been designed to solve a modified Beer-Lambert equation and to measure changes in the concentrations of oxygenated and deoxygenated hemoglobins.
Despite their capability of providing the hemoglobin concentrations as well as the oxygen saturation, one major drawback of TRS and PMS is that the equipment is bulky and expensive. CWS may be manufactured at a lower cost but is limited in its utility because it cannot compute the oxygen saturation from the changes in the concentrations of oxygenated and deoxygenated hemoglobins.
Optical Diffusion Imaging and Spectroscopy (ODIS) allows tissue to be characterized based on measurements of photon scattering and absorption. In tissue such as human tissue, near infrared light is highly scattered and minimally absorbed. Optical diffusion imaging is achieved by sending optical signals into tissue and measuring the corresponding diffuse reflectance or transmittance on the tissue surface.
Scattering is caused by the heterogeneous structure of a tissue and, therefore, is an indicator of the density of a cell and the nuclear size of the cell. Absorption is caused by interaction with chromophores. ODIS emits light into tissue through a sensor. The position of the light source which emits the light and a detector which detects the light allows a depth of measurement to be determined. A ratio of oxyhemoglobin and deoxyhemoglobin may be used to allow for substantially real-time measurement of oxygen, e.g., oxygen saturation levels.
Within ODIS systems, sensors which come into contact with tissue surfaces generally have optical fibers arranged thereon in a substantially symmetric layout. That is, optical fibers that are coupled to light sources are arranged in a substantially symmetric orientation relative to optical fibers that are coupled to light detectors. While a symmetric orientation is effective in allowing for oxygen saturation levels to be measured, the manufacture of such sensor is often difficult, as the exact placement of the optical fibers within the sensor is crucial. Further, when the anatomy of tissue or underlying structure is not substantially symmetric, the use of a sensor with a symmetric orientation may not allow for accurate measurements to be readily made.
Therefore, what is needed is a sensor that is relatively easy to manufacture, and is arranged to be used on tissue which may not have a symmetric anatomy. That is, what is desired is a sensor with a layout of optical fibers for light sources and optical fibers for detectors that facilitates use with tissue having substantially any anatomy.